There are currently four industry accepted types of dehumidification. They are: 1. Compression of the air. This will reduce the absolute moisture content of the air but will generally produce a saturated condition at the elevated pressure. Expansion of this high pressure air will result in a lower dew point at the lower pressure because of the increase in actual volume. This is similar to what one experiences with an air compressor. The removal of the condensed water is accomplished by use of water traps and after coolers. However, the amount of air treated does not make this a viable alternative for dehumidification within the industrial marketplace.

2. Liquid sorption. The air is passed through sprays of liquid sorbent, such as lithium chloride or glycol solution. The sorbent in its active state has a vapor pressure below that of the air being dehumidified and thus absorbs moisture from the air stream. The sorbent must be continually regenerated by using heat to drive off the absorbed moisture.

3. Solid sorption (desiccant). This method utilizes either granular beds or fixed desiccant structures that are employed in automatic machines through which the air is passed. This desiccant also needs to be reactivated by heat to release the previously sorbed moisture to an outdoor stream.

4. Condensation-based (Refrigerant). This type unit, pictured above, chills the air below its dew point, causing moisture to form as condensation on the cold surface of the cooling coil and thus removes water from the air.

In practicality, from the standpoint of the corrosion control and product protection industries, only the condensation based (refrigerant) and the solid sorption/desiccant types are applicable.

1. The desiccant based dehumidification system uses a chemical to directly absorb moisture from the air while it is a vapor. Specifically, the moist air stream is passed over a desiccant, typically lithium chloride or silica gel, that in its active state has a vapor pressure below that of the air to be dehumidified. Moisture is absorbed from the air stream. The desiccant is then heated which forces it to give up the absorbed moisture, regenerating the desiccant for continuous use. The heat of regeneration causes the temperature of the air entering the enclosure to be substantially higher than the ambient air. Due to this heat of regeneration requirement, the power requirements to operate this type of unit are generally quite high. Ultimately the desiccant will have to be completely replaced to maintain its performance level.

2. The condensation based (refrigerant) dehumidification system has the incoming air cross over the evaporator coils to reduce the absolute amount of moisture in the air via condensation. The air exits the cooling coil section of the dehumidifier at a reduced temperature, dew point, and absolute humidity. It then passes over both the condenser coils and a series of reheat coils to (a) increase the temperature of the air and (b) reduce the relative humidity of this air.

This system is advantageous when the ambient external air is comparatively warm with a high moisture content and the dew point is greater than 0 degrees C (32 degrees F). It has low power consumption requirements - approximately half that of a desiccant unit with an equal air flow rating.

Enviro-Air Control Corp., 1523 N. Post Oak Rd., Houston, TX 77055-8409. Tel: 713-681-3449; Fax: 713-688-8273.

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